In some 'Danny Phantom' fanfictions, the scars can be a way to explore Danny's character growth. A new scar might make him more cautious in battles, or it could make him more determined to get stronger so that he doesn't get injured as easily in the future. They can also be a way for other characters to show their concern for Danny. For example, his friends might notice a new scar and try to help him in different ways, like finding a way to heal it or making sure he has better protection in future battles.
It's hard to give an exact length for the JJK manga as it's still ongoing. But it's been running for a while and has built up a substantial amount of content.